Modern radio frequency ("RF") communication systems such as, for example, the "iDEN" system which is commerciallly available from Motorola, Inc. of Schaumburg, Ill., have one or more cells. As is known, each cell is served by a corresponding cell site, with each cell site being controlled by a controller unit. In high-performance systems, each cell site contains redundant controllers. With this arrangement, one controller is on-line (the "on-line controller"), while the other controller is off-line (the "off-line controller"). If the on-line controller fails, the off-line controller is arranged to take over control of the cell site.
In the past, each of the multiple controllers was coupled to the RF system backbone by means of a dedicated link such as, for example, a T1 link. Obviously, for multiple controllers, this arrangement requires multiple links for each cell site. One problem with this arrangement, of course, is the cost of the multiple links.
To remedy the problem of the multiple links, in the past a single link has been provided to the cell site, with the single link being split into two (2) parallel branches, each branch then being coupled to one of the two controllers. Further, as above, the controllers were programmed with a software algorithm so that, when the on-line controller failed, it would "pass off" control to the off-line controller. The off-line controller would then take over and operate the cell site.
It is known that a cell site controller typically includes one or more peripheral units arranged to support the controller. Accordingly, the proper functioning of a cell site depends not only on the proper functioning of the cell site's on-line controller but also on the proper functioning of the one or more peripheral units arranged to support the cell site's on-line controller.
One problem with the prior switching method was that it switched the cell site from the on-line controller to the off-line controller based solely on whether the on-line controller itself failed, and without regard to the status of any peripheral units arranged to support the on-line controller. This problem is illustrated below.
One example of a peripheral unit that is typically arranged to support an on-line controller is a time frequency reference ("TFR") unit. It is known that an on-line controller can continue to operate for some time after the failure of its time frequency reference unit. As a result of the prior switching method, therefore, the on-line controller would continue to control the cell site even though its time frequency reference has failed. In this case, the cell site's timing ultimately will drift, then fail, probably within 30-40 minutes, thus bringing down the entire cell site.
In the foregoing example it must be appreciated that whilst the on-line controller continues to operate with its failed time frequency reference, at all times there remain available for use the off-line controller with its corresponding good time frequency reference. However, since the prior switching method was based solely on the failure of the on-line controller itself and without regard to the status of the on-line controller's supporting peripheral units, the cell site was deprived of the opportunity to switch to the off-line controller in a timely manner.
In summary, there is a need for an improved method for switching cell site controllers.